Apparatus of plural charged-particle beams

ABSTRACT

A new multi-beam apparatus with a total FOV variable in size, orientation and incident angle, is proposed. The new apparatus provides more flexibility to speed the sample observation and enable more samples observable. More specifically, as a yield management tool to inspect and/or review defects on wafers/masks in semiconductor manufacturing industry, the new apparatus provide more possibilities to achieve a high throughput and detect more kinds of defects.

CLAIM OF PRIORITY

This application is a reissue of U.S. Pat. No. 10,062,541 B2, issued onAug. 28, 2018, from U.S. patent application Ser. No. 15/417,360, filedJan. 27, 2017, which claims the benefit of priority of U.S. provisionalapplication No. 62/287,626 entitled to Ren et al. filed on Jan. 27, 2016and entitled “Apparatus of Plural Charged-Particle Beams”, the entiredisclosures of which are incorporated herein by reference. The contentsof the above-identified applications are incorporated herein byreference in their entireties.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. application Ser. No. 15/065,342entitled to Ren et al. filed on Mar. 9, 2016 and entitled “Apparatus ofPlural Charged-Particle Beams”, the entire disclosures of which areincorporated herein by reference

This application is related to U.S. application Ser. No. 15/078,369entitled to Ren et al. filed on Mar. 23, 2016 and entitled “Apparatus ofPlural Charged-Particle Beams”, the entire disclosures of which areincorporated herein by reference.

This application is related to U.S. application Ser. No. 15/150,858entitled to Liu et al. filed on May 10, 2016 and entitled “Apparatus ofPlural Charged-Particle Beams”, the entire disclosures of which areincorporated herein by reference.

This application is related to U.S. application Ser. No. 15/213,781entitled to Li et al. filed on Jul. 19, 2016 and entitled “Apparatus ofPlural Charged-Particle Beams”, the entire disclosures of which areincorporated herein by reference.

This application is related to U.S. application Ser. No. 15/216,258entitled to Ren et al. filed on Jul. 21, 2016 and entitled “Apparatus ofPlural Charged-Particle Beams”, the entire disclosures of which areincorporated herein by reference.

This application is related to U.S. application Ser. No. 15/365,145entitled to Ren et al. filed on Nov. 30, 2016 and entitled “Apparatus ofPlural Charged-Particle Beams”, the entire disclosures of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus with a plurality ofcharged-particle beams. More particularly, it relates to an apparatuswhich employs plural charged-particle beams to simultaneously acquireimages of plural scanned regions within an observed area on a samplesurface. Hence, the apparatus can be used to inspect and/or reviewdefects on wafers/masks with high resolution and high throughput insemiconductor manufacturing industry.

2. Description of the Prior Art

The following description and examples are not admitted to be prior artby their mention in this Background section. For manufacturingsemiconductor IC chips, pattern defects and/or uninvited particles(residuals) inevitably appear on surfaces of wafers/mask duringfabrication processes, which reduce the yield to a great degree.Accordingly, the yield management tools are used to inspect and/orreview the defects and the particles. To meet the more and more advancedrequirements on performance of IC chips, the patterns with smaller andsmaller critical feature dimensions have been adopted. Consequently, theconventional yield management tools with optical beam gradually becomeincompetent due to diffraction effect, and the yield management toolswith electron beam are more and more employed. Compared to a photonbeam, an electron beam has a shorter wavelength and thereby possiblyoffering superior spatial resolution. Currently, the yield managementtools with electron beams employ the principle of scanning electronmicroscope (SEM) with a single electron beam, and as well known theirthroughputs are not competent for mass production. Although increasingthe beam currents can improve the throughputs, the superior spatialresolutions will be fundamentally deteriorated by Coulomb Effect whichincreases with the beam currents.

For mitigating the limitation on throughput, instead of using a singleelectron beam with a large current, a promising solution is to use aplurality of electron beams each with a small current. The plurality ofelectron beams forms a plurality of probe spots or simply called as aprobe spot array on one being-inspected or observed surface of a sample.The plurality of probe spots can respectively and simultaneously scan aplurality of small scanned regions within a large observed area on thesample surface. The electrons of each probe spot generate secondaryelectrons from the sample surface where they land on. The secondaryelectrons comprise slow secondary electrons (energies ≤50 eV) andbackscattered electrons (energies close to landing energies of theelectrons). The secondary electrons from the plurality of small scannedregions can be respectively and simultaneously collected by a pluralityof electron detectors. Consequently, the image of the large observedarea including all of the small scanned regions can be obtained muchfaster than scanning the large observed area with a single beam.

The plurality of electron beams can be either from a plurality ofelectron sources respectively, or from a single electron source. For theformer, the plurality of electron beams is usually focused onto andscans the plurality of small scanned regions by a plurality of columnsrespectively, and the secondary electrons from each scanned region aredetected by one electron detector inside the corresponding column. Theapparatus therefore is generally called as a multi-column apparatus. Onthe sample surface, the beam interval or pitch is on the order ofseveral to tens millimeters.

For the latter, a source-conversion unit virtually changes the singleelectron source into a plurality of sub-sources. The source-conversionunit comprises one beamlet-limit (or beamlet-forming) means with aplurality of beam-limit openings and one image-forming means with aplurality of electron optics elements. The plurality of beam-limitopenings divides the primary-electron beam generated by the singleelectron source into a plurality of sub-beams or beamlets respectively,and the plurality of electron optics elements influence the plurality ofbeamlets to form a plurality of first parallel (virtual or real) imagesof the single electron source respectively. Each first image is thecross-over of one beamlet and can be taken as one sub-source which emitsthe corresponding beamlet. To make more beamlets available, the beamletintervals are at micro meter level. Naturally, one primary projectionimaging system and one deflection scanning unit within one single columnare used to project the plurality of first parallel images onto and scanthe plurality of small scanned regions respectively. The plurality ofsecondary electron beams therefrom is directed by one beam separatorinto one secondary projection imaging system, and then focused by thesecondary projection imaging system to be respectively detected by aplurality of detection elements of one electron detection device insidethe single column. The plurality of detection elements can be aplurality of electron detectors placed side by side or a plurality ofpixels of one electron detector. The apparatus therefore is generallycalled as a multi-beam apparatus

The beamlet-limit means is usually an electric-conduction plate withthrough-holes, and a plurality of through-holes therein functions theplurality of beam-limit openings respectively. For the image-formingmeans, each electron optics element either focuses one beamlet to formone real image (such as U.S. Pat. No. 7,244,949 and the fourth relatedapplication in the CROSS REFERENCE), or deflects one beamlet to form onevirtual image (such as U.S. Pat. No. 6,943,349 and the other relatedapplications in the CROSS REFERENCE). FIG. 1A and FIG. 1B show twoexamples in the fifth related application. For sake of clarity, onlythree beamlets are shown, and the deflection scanning unit, the beamseparator, the secondary projection imaging system and the electrondetection device are not shown.

In FIG. 1A, the primary-electron beam 102 generated by the electronsource 101 is focused by the condenser lens 110 to be incident onto thesource-conversion unit 120. The source-conversion unit 120 comprises onepre-beamlet-bending means 123 with three pre-bending micro-deflectors123_1, 123_2 and 123_3, one beamlet-limit means 121 with threebeam-limit openings 121_1, 121_2 and 121_3 and one image-forming means122 with three electron optics elements 122_1, 122_2 and 122_3. Thethree pre-bending micro-deflectors 123_1˜123_3 respectively deflect thethree beamlets 102_1˜102_3 perpendicularly incident onto the threebeam-limit openings 121_1˜121_3, and each of which functions as abeam-limit aperture to limit the current of the corresponding beamlet.The three electron optics elements 122_1˜122_3 deflects the threebeamlets 102_1˜102_3 towards the primary optical axis 100_1 and formthree first virtual images of the electron source 101, i.e. each beamlethas a virtual crossover. The objective lens constitutes the primaryprojection imaging system, which focuses the three deflected beamlets102_1˜102_3 onto the surface 7 of the sample 8, i.e. projecting thethree first virtual images thereon. The three beamlets 102_1˜102_3therefore form three probe spots 102_1s, 102_2s and 102_3s on thesurface 7. The currents of the probe spots 102_1s˜102_3s can be variedby adjusting the focusing power of the condenser lens 110. In FIG. 1B,the movable condenser lens 210 focuses the primary-electron beam 102 tobe normally incident onto the beamlet-limit means 121 of thesource-conversion unit 220, and the pre-beamlet-bending means 123 inFIG. 1A is therefore not needed. Accordingly the currents of the probespots 102_1s-102_3s can be varied by adjusting the focusing power andthe position of the movable condenser lens 210. In FIG. 1A and FIG. 1B,the landing energies of the beamlets 102_1˜102_3 on the sample surface 7can be varied by adjusted either or both of the potentials of theelectron source 101 and the sample surface 7.

In a multi-beam apparatus, each beamlet scans one sub-FOV (field ofview) on the sample surface, and the total FOV is the sum of thesub-FOVs of the plural beamlets. Each sub-FOV is equal or smaller thanthe beamlet pitch on the sample surface (Ps in FIG. 1A). To furtherimprove the throughput, each sub-FOV is better selectable in terms ofthe imaging resolution and the pitches of the plural beamlets areaccordingly varied to keep the sub-FOVs stitched up. In one case withhigh image resolution, a small pixel size will be used and a smallsub-FOV is desired for avoiding a large pixel number. In another casewith low image resolution, a large pixel size will be used and a largesub-FOV is desired for a high throughput. FIG. 2A shows an example inthe later case. As shown in dash line, if the probe spots 102_2s and102_3s in FIG. 1A can be intentionally moved to the right and the leftrespectively, i.e. the pitch Ps can be changed from P1 to P2, the totalFOV will be increased from 3×P1 to 3×P2, and accordingly the throughputis increased. Hence making the beamlet pitch Ps selectable will be onepreferred function.

The continuous scanning mode (a sample continuously moving in thedirection perpendicular to a scanning direction of a primary electronbeam) is a conventional method to get high throughput in a conventionalsingle-beam apparatus. If using this method in a multi-beam apparatus,it is better to match the orientation of the total FOV or the probe spotarray with the stage moving direction. As well known, if there is onemagnetic lens in the primary projection imaging system, the magneticfield thereof will rotate the plural beamlets and the total FOV as aresult. Due to the magnetic field is varied with respect to theobserving conditions (such as landing energies and currents of pluralbeamlets), the rotation angle of the total FOV will accordingly vary.FIG. 2B shows an example if the objective lens 131 in FIG. 1A is amagnetic or electromagnetic compound lens. For instance, when thelanding energies of the beamlets 102_1˜102_3 are changed from 1 keV to 2keV, the probe spots 102_2s and 102_3s will rotate an angle β around theoptical axis 100_1 as shown in dash line, i.e. the orientation of thetotal FOV rotates the angle β. The orientation variation of the totalFOV impacts the performance of the continuous scanning mode. Keeping theorientation of the probe spot array same or making it selectable canprovide more flexibility to improve the throughput, and accordingly isanother preferred function.

For some sample, a specific match between the orientations of patternsthereon and the probe spot array may be required. Making the orientationof the probe spot array selectable can compensate the mismatch due tothe limited loading accuracy, and therefore can increase the throughputby avoiding the time-consuming of re-loading. In addition, toeffectively observe some patterns of a sample, the plural beamlets maybe required to land onto the sample surface with specific incidentangles. Making the incident angles selectable can enable more samples orpatterns observable, and will be one more preferred function.

The present invention will provide methods to realize the foregoingfunctions in a multi-beam apparatus, especially for those proposed inthe CROSS REFERENCE and used as yield management tools in semiconductormanufacturing industry.

SUMMARY OF THE INVENTION

The object of this invention is to provide a new multi-beam apparatusfor observing a sample with high resolution and high throughput and inflexibly varying observing conditions. Based on the conventionalmulti-beam apparatuses in the CROSS REFERENCE, this invention proposesseveral methods to configure the new multi-beam apparatus with avariable total FOV. In the new apparatus, the total FOV can be variablein size, orientation and incident angle. Hence the new apparatusprovides more flexibility to speed the sample observation and enablemore kinds of samples observable. More specifically, if being used as ayield management tool in semiconductor manufacturing industry to inspectand/or review defects on wafers/masks, the new apparatus can providemore possibilities to achieve a high throughput and detect more kinds ofdefects.

Accordingly, the invention therefore provides a multi-beam apparatus forobserving a surface of a sample, which comprises an electron source, acondenser lens below the electron source, a source-conversion unit belowthe condenser lens, an objective lens below the source-conversion unit,a deflection scanning unit below the source-conversion unit, a samplestage below the objective lens, a beam separator below thesource-conversion unit, a secondary projection imaging system, and anelectron detection device with a plurality of detection elements. Theelectron source, the condenser lens and the objective lens are alignedwith a primary optical axis of the apparatus, and the sample stagesustains the sample so that the surface faces to the objective lens. Thesource-conversion unit comprises a beamlet-limit means with a pluralityof beam-limit openings, and an image-forming means with a plurality ofelectron optics elements and movable along the primary optical axis. Theelectron source generates a primary-electron beam along the primaryoptical axis and the condenser lens focuses the primary-electron beam. Aplurality of beamlets of the primary-electron beam pass through theplurality of beam-limit openings respectively, and is deflected by theplurality of electron optics elements towards the primary optical axisto form a plurality of virtual images of the electron sourcerespectively. The plurality of beamlets is focused by the objective lensonto the surface and therefore forms a plurality of probe spots thereonrespectively, and the deflection scanning unit deflects the plurality ofbeamlets to scan the plurality of probe spots respectively over aplurality of scanned regions within an observed area on the surface. Aplurality of secondary electron beams is generated by the plurality ofprobe spots respectively from the plurality of scanned regions anddirected into the secondary projection imaging system by the beamseparator, the secondary projection imaging system focuses and keeps theplurality of secondary electron beams to be detected by the plurality ofdetection elements respectively, and each detection element thereforeprovides an image signal of one corresponding scanned region.

Deflection angles of the plurality of beamlets due to the plurality ofelectron optics elements are respectively set to reduce off-axisaberrations of the plurality of probe spots. Pitches of the plurality ofprobe spots are varied together by moving the image-forming means alongthe primary optical axis. The objective lens comprises a magnetic lensand an electrostatic lens. An orientation of the plurality of probespots is selectable by varying a ratio of focusing powers of themagnetic lens and the electrostatic lens.

The deflection angles may ensure the plurality of beamlets to land onthe surface perpendicularly or substantially perpendicularly. Thedeflection angles may ensure the plurality of beamlets to obliquely landon the surface with same or substantially same landing angles. Thedeflection scanning unit is above a front focal plane of the objectivelens. The deflection scanning unit tilts the plurality of beamlets toobliquely land on the surface with same or substantially same landingangles. The apparatus may further comprise a beamlet-tilting deflectorbetween the source-conversion unit and a front focal plane of theobjective lens. The beamlet-tilting deflector tilts the plurality ofbeamlets to obliquely land on the surface with same or substantiallysame landing angles.

The invention also provides a multi-beam apparatus for observing asurface of a sample, which comprises an electron source, a condenserlens below the electron source, a source-conversion unit below thecondenser lens, an objective lens below the source-conversion unit, adeflection scanning unit below the source-conversion unit, a samplestage below the objective lens, a beam separator below thesource-conversion unit, a secondary projection imaging system, and anelectron detection device with a plurality of detection elements. Theelectron source, the condenser lens and the objective lens are alignedwith a primary optical axis of the apparatus, and the sample stagesustains the sample so that the surface faces to the objective lens. Thesource-conversion unit comprises a beamlet-limit means with a pluralityof beam-limit openings, a first image-forming means with a plurality offirst electron optics elements and a second image-forming means with aplurality of second electron optics elements, the second image-formingmeans is below the first image-forming means and movable in a radialdirection, and one of the first image-forming means and the secondimage-forming means is used as an active image-forming means. Theelectron source generates a primary-electron beam along the primaryoptical axis and the condenser lens focuses the primary-electron beam. Aplurality of beamlets of the primary-electron beam pass through theplurality of beam-limit openings respectively, and is deflected by theactive image-forming means towards the primary optical axis to form aplurality of virtual images of the electron source respectively. Theplurality of beamlets is focused by the objective lens onto the surfaceand therefore forms a plurality of probe spots thereon respectively, andthe deflection scanning unit deflects the plurality of beamlets to scanthe plurality of probe spots respectively over a plurality of scannedregions within an observed area on the surface. A plurality of secondaryelectron beams is generated by the plurality of probe spots respectivelyfrom the plurality of scanned regions and directed into the secondaryprojection imaging system by the beam separator, the secondaryprojection imaging system focuses and keeps the plurality of secondaryelectron beams to be detected by the plurality of detection elementsrespectively, and each detection element therefore provides an imagesignal of one corresponding scanned region.

Deflection angles of the plurality of beamlets due to the activeimage-forming means are respectively set to reduce off-axis aberrationsof the plurality of probe spots. Pitches of the plurality of probe spotsare varied together by changing the active image-forming means betweenthe first image-forming means and the second image-forming means, andwhen the first image-forming means is selected, the second image-formingmeans is moved outside so as not to block the plurality of beamlets. Theobjective lens comprises a magnetic lens and an electrostatic lens. Anorientation of the plurality of probe spots is selectable by varying aratio of focusing powers of the magnetic lens and the electrostaticlens.

The deflection angles may ensure the plurality of beamlets to land onthe surface perpendicularly or substantially perpendicularly. Thedeflection angles may ensure the plurality of beamlets to obliquely landon the surface with same or substantially same landing angles. Thedeflection scanning unit is above a front focal plane of the objectivelens. The deflection scanning unit tilts the plurality of beamlets toobliquely land on the surface with same or substantially same landingangles. The apparatus may further comprise a beamlet-tilting deflectorbetween the source-conversion unit and a front focal plane of theobjective lens. The beamlet-tilting deflector tilts the plurality ofbeamlets to obliquely land on the surface with same or substantiallysame landing angles.

The invention also provides a multi-beam apparatus for observing asurface of a sample, which comprises an electron source, a condenserlens below the electron source, a source-conversion unit below thecondenser lens, an objective lens below the source-conversion unit, adeflection scanning unit below the source-conversion unit, a samplestage below the objective lens, a beam separator below thesource-conversion unit, a secondary projection imaging system, and anelectron detection device with a plurality of detection elements. Theelectron source, the condenser lens and the objective lens are alignedwith a primary optical axis of the apparatus, a first principal plane ofthe objective lens is movable along the primary optical axis, and thesample stage sustains the sample so that the surface faces to theobjective lens. The source-conversion unit comprises a beamlet-limitmeans with a plurality of beam-limit openings, and an image-formingmeans with a plurality of electron optics elements. The electron sourcegenerates a primary-electron beam along the primary optical axis and thecondenser lens focuses the primary-electron beam. A plurality ofbeamlets of the primary-electron beam pass through the plurality ofbeam-limit openings respectively, and is deflected by the plurality ofelectron optics elements towards the primary optical axis to form aplurality of virtual images of the electron source respectively. Theplurality of beamlets is focused by the objective lens onto the surfaceand therefore forms a plurality of probe spots thereon respectively, andthe deflection scanning unit deflects the plurality of beamlets to scanthe plurality of probe spots respectively over a plurality of scannedregions within an observed area on the surface. A plurality of secondaryelectron beams is generated by the plurality of probe spots respectivelyfrom the plurality of scanned regions and directed into the secondaryprojection imaging system by the beam separator, the secondaryprojection imaging system focuses and keeps the plurality of secondaryelectron beams to be detected by the plurality of detection elementsrespectively, and each detection element therefore provides an imagesignal of one corresponding scanned region.

Deflection angles of the plurality of beamlets due to the plurality ofelectron optics elements are respectively set to reduce off-axisaberrations of the plurality of probe spots. Pitches of the plurality ofprobe spots are varied together by moving the first principal planealong the primary optical axis.

The deflection angles may ensure the plurality of beamlets to land onthe surface perpendicularly or substantially perpendicularly. Thedeflection angles may ensure the plurality of beamlets to obliquely landon the surface with same or substantially same landing angles. Thedeflection scanning unit is above a front focal plane of the objectivelens. The deflection scanning unit tilts the plurality of beamlets toobliquely land on the surface with same or substantially same landingangles. The apparatus may further comprise a beamlet-tilting deflectorbetween the source-conversion unit and a front focal plane of theobjective lens. The beamlet-tilting deflector tilts the plurality ofbeamlets to obliquely land on the surface with same or substantiallysame landing angles. The objective lens comprises a lower magnetic lensand an electrostatic lens. The electrostatic lens comprises afield-control electrode and a field-moving electrode, and generates anelectrostatic field. A potential of the field-control electrode is setto control the electrostatic field on the surface for the sample free ofelectrical breakdown. A potential of the field-moving electrode is setto move the electrostatic field for moving the first principal plane. Anorientation of the plurality of probe spots is selectable by varyingeither or both of potentials of the field-control electrode and thefield-moving electrode. The apparatus may further comprise an uppermagnetic lens above the lower magnetic lens. The first principal planeis moved by varying a ratio of focusing powers of the lower magneticlens and the upper magnetic lens. An orientation of the plurality ofprobe spots is selectable by setting polarities of magnetic fields ofthe upper and lower magnetic lenses same or opposite.

The invention also provides a multi-beam apparatus for observing asurface of a sample, which comprises an electron source, a condenserlens below the electron source, a source-conversion unit below thecondenser lens, a transfer lens below the source-conversion unit, afield lens below the transfer lens, an objective lens below the fieldlens, a deflection scanning unit below the source-conversion unit, asample stage below the objective lens, a beam separator below thesource-conversion unit, a secondary projection imaging system, and anelectron detection device with a plurality of detection elements. Theelectron source, the condenser lens, the transfer lens, the field lensand the objective lens are aligned with a primary optical axis of theapparatus, and the sample stage sustains the sample so that the surfacefaces to the objective lens. The source-conversion unit comprises abeamlet-limit means with a plurality of beam-limit openings, and animage-forming means with a plurality of electron optics elements. Theelectron source generates a primary-electron beam along the primaryoptical axis and the condenser lens focuses the primary-electron beam. Aplurality of beamlets of the primary-electron beam pass through theplurality of beam-limit openings respectively, and is deflected by theplurality of electron optics elements towards the primary optical axisto form a plurality of first virtual images of the electron sourcerespectively. The transfer lens images the plurality of first virtualimages onto an intermediate image plane and therefore forms a pluralityof second real images respectively thereon, the field lens is placed onthe intermediate image plane and bends the plurality of beamlets, theobjective lens images the plurality of second real images onto thesurface and therefore forms a plurality of probe spots thereonrespectively, and the deflection scanning unit deflects the plurality ofbeamlets to scan the plurality of probe spots respectively over aplurality of scanned regions within an observed area on the surface. Aplurality of secondary electron beams is generated by the plurality ofprobe spots respectively from the plurality of scanned regions anddirected into the secondary projection imaging system by the beamseparator, the secondary projection imaging system focuses and keeps theplurality of secondary electron beams to be detected by the plurality ofdetection elements respectively, and each detection element thereforeprovides an image signal of one corresponding scanned region.

Bending angles of the plurality of beamlets due to the field lens areset to reduce off-axis aberrations of the plurality of probe spots.Deflection angles of the plurality of beamlets due to the plurality ofelectron optics elements are adjusted to change pitches of the pluralityof probe spots respectively. The objective lens comprises a firstmagnetic lens and a first electrostatic lens. An orientation of theplurality of probe spots is selectable by varying a ratio of focusingpowers of the first magnetic lens and the first electrostatic lens. Thetransfer lens comprises a second magnetic lens and a secondelectrostatic lens. An orientation of the plurality of probe spots isselectable by varying a ratio of focusing powers of the second magneticlens and the second electrostatic lens. The field lens comprises a thirdmagnetic lens and a third electrostatic lens. An orientation of theplurality of probe spots is selectable by varying a ratio of focusingpowers of the third magnetic lens and the third electrostatic lens. Thebending angles and deflection angles of the plurality of beamlets due tothe plurality of electron optics elements may ensure the plurality ofbeamlets to land on the surface perpendicularly or substantiallyperpendicularly. The bending angles and deflection angles of theplurality of beamlets due to the plurality of electron optics elementsmay ensure the plurality of beamlets to obliquely land on the surfacewith same or substantially same landing angles. The deflection scanningunit is above a front focal plane of the objective lens. The deflectionscanning unit tilts the plurality of beamlets to obliquely land on thesurface with same or substantially same landing angles. The apparatusmay further comprise a beamlet-tilting deflector between thesource-conversion unit and a front focal plane of the objective lens.The beamlet-tilting deflector tilts the plurality of beamlets toobliquely land on the surface with same or substantially same landingangles.

The invention also provides a method to configure a multi-beam apparatusfor observing a surface of a sample, which comprises steps ofconfiguring an image-forming means of a source-conversion unit movablealong a primary optical axis thereof; using the image-forming means toform a plurality of virtual images of an electron source respectively;using an objective lens to image the plurality of virtual images ontothe surface and form a plurality of probe spots thereon; and moving theimage-forming means to vary pitches of the plurality of probe spots.

The invention also provides a method to configure a multi-beam apparatusfor observing a surface of a sample, which comprises steps ofconfiguring a source-conversion unit with a first image-forming meansand a second image-forming means, wherein the second image-forming meansis farther away from an electron source than the first image-formingmeans and movable in a radial direction of the apparatus; using one ofthe first image-forming means and the second image-forming means as anactive image-forming means, wherein when the first image-forming meansis used, the second image-forming means is moved away; using the activeimage-forming means to form a plurality of virtual images of theelectron source respectively; using an objective lens to image theplurality of virtual images onto the surface and form a plurality ofprobe spots thereon; and changing the active image-forming means betweenthe first image-forming means and the second image-forming means to varypitches of the plurality of probe spots.

The invention also provides a method to configure a multi-beam apparatusfor observing a surface of a sample, which comprises steps ofconfiguring an objective lens with a first principal plane movable alonga primary optical axis of the apparatus; using an image-forming means ofa source- conversion unit to form a plurality of virtual images of anelectron source respectively; using the objective lens to image theplurality of virtual images onto the surface and form a plurality ofprobe spots thereon; and moving the first principal plane to varypitches of the plurality of probe spots.

The invention also provides a method to configure a multi-beam apparatusfor observing a surface of a sample, which comprises steps ofconfiguring an objective lens with a lower magnetic lens and anelectrostatic lens in the apparatus; using an image-forming means of asource-conversion unit to form a plurality of virtual images of anelectron source respectively; using the objective lens to image theplurality of virtual images onto the surface and form a plurality ofprobe spots thereon; and changing a ratio of focusing powers of themagnetic lens and the electrostatic lens to select an orientation of theplurality of probe spots.

The method may further comprise a step of configuring the objective lenswith an upper magnetic lens farther away from the surface than the lowermagnetic lens. The method may further comprise a step of changingpolarities of magnetic fields of the upper and lower magnetic lenses toselect the orientation.

The invention also provides a method to configure a multi-beam apparatusfor observing a surface of a sample, which comprises steps of using animage-forming means of a source-conversion unit to deflect a pluralityof beamlets from an electron source to form a plurality of first virtualimages thereof respectively; using an objective lens to image theplurality of virtual images onto the surface and form a plurality ofprobe spots thereon; and setting deflection angles of the plurality ofbeamlets due to the image-forming means so that the plurality ofbeamlets lands on the surface with same or substantially same landingangles.

The method may further comprise a step of changing the deflection anglesto equally vary the landing angles. The method may further comprise astep of using a deflection scanning unit to tilt the plurality ofbeamlets so as to equally vary the landing angles. The method mayfurther comprise a step of using a beamlet-tilting deflector to tilt theplurality of beamlets so as to equally vary the landing angles.

The invention also provides a method to configure a multi-beam apparatusfor observing a surface of a sample, which comprises steps of using animage-forming means of a source-conversion unit to deflect a pluralityof beamlets from an electron source to form a plurality of first virtualimages thereof respectively; using a transfer lens to image theplurality of first virtual images onto an intermediate image plane andforms a plurality of second real images respectively; placing a fieldlens on the intermediate image plane to bend the plurality of beamlets;and using an objective lens to image the plurality of second real imagesonto the surface and form a plurality of probe spots thereon.

The method may further comprise a step of changing deflection angles ofthe plurality of beamlets due to the image-forming means to vary pitchesof the plurality of probe spots. The method may further comprise a stepof setting deflection angles of the plurality of beamlets due to theimage-forming means and bending angles of the plurality of beamlets dueto the field lens so that the plurality of beamlets lands on the surfacewith same or substantially same landing angles. The method may furthercomprise a step of varying the deflection angles to equally change thelanding angles. The method may further comprise a step of using adeflection scanning unit to tilt the plurality of beamlets to equallychange the landing angles. The method may further comprise a step ofusing a beamlet-tilting deflector to tilt the plurality of beamlets toequally change the landing angles. The method may further comprise astep of configuring the objective lens with a first magnetic lens and afirst electrostatic lens. The method may further comprise a step ofchanging a ratio of focusing powers of the first magnetic lens and thefirst electrostatic lens to select an orientation of the plurality ofprobe spots. The method may further comprise a step of configuring thetransfer lens with a second magnetic lens and a second electrostaticlens. The method may further comprise a step of changing a ratio offocusing powers of the second magnetic lens and the second electrostaticlens to select an orientation of the plurality of probe spots. Themethod may further comprise a step of configuring the field lens with athird magnetic lens and a third electrostatic lens. The method mayfurther comprise a step of changing a ratio of focusing powers of thethird magnetic lens and the third electrostatic lens to select anorientation of the plurality of probe spots.

The invention also provides an apparatus, which comprises a source forproviding a primary charged particle beam, a source-conversion unit fordividing the primary charged particle beam into a plurality of chargedparticle beamlets and using which to form a plurality of images of thesource respectively, and an objective lens below the source-conversionunit for projecting the plurality of images onto a sample surface.Pitches of the plurality of charged particle beamlets on the samplesurface are adjustable by changing deflection angles of the plurality ofcharged particle beamlets prior entering the objective lens.

The invention also provides an apparatus, which comprises a source forproviding a primary charged particle beam, means for using a pluralityof beamlets of the primary charged particle beam to form a plurality ofimages of the source, an objective lens for projecting the plurality ofimages onto a sample surface to form a plurality of probe spots, andmeans for adjusting pitches of the plurality of probe spots on thesample surface.

The invention also provides a method for observing a sample surface,which comprises steps of providing a plurality of charged particle beamswith a plurality of crossovers respectively, projecting the plurality ofcrossovers onto the sample surface to form a plurality of probe spotsthereon, scanning the plurality of probe spots on the sample surface,and changing deflection angles of the plurality of charged particlebeams such that pitches of the plurality of spots can be adjusted.

Other advantages of the present invention will become apparent from thefollowing description taken in conjunction with the accompanyingdrawings wherein are set forth, by way of illustration and example,certain embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings,wherein like reference numerals designate like structural elements, andin which:

FIGS. 1A and 1B are schematic illustrations of two configurations of theconventional multi-beam apparatus disclosed in the fifth application ofthe CROSS REFERENCE.

FIG. 1C is a schematic illustration of one configuration of aconventional electromagnetic compound objective lens.

FIGS. 2A and 2B are schematic illustrations of the total FOV varying insize and orientation.

FIG. 3A is a schematic illustration of one configuration of the newmulti-beam apparatus in accordance with one embodiment of the presentinvention.

FIGS. 3B and 3C are schematic illustrations of varying the size of thetotal FOV in accordance with the embodiment in FIG. 3A.

FIG. 4A is a schematic illustration of another configuration of the newmulti-beam apparatus in accordance with another embodiment of thepresent invention.

FIGS. 4B and 4C are schematic illustrations of varying the size of thetotal FOV in accordance with the embodiment in FIG. 4A.

FIG. 5A is a schematic illustration of another configuration of the newmulti-beam apparatus in accordance with another embodiment of thepresent invention.

FIGS. 5B and 5C are schematic illustrations of varying the size of thetotal FOV in accordance with the embodiment in FIG. 5A.

FIG. 6A is a schematic illustration of another configuration of the newmulti-beam apparatus in accordance with another embodiment of thepresent invention.

FIGS. 6B and 6C are schematic illustrations of varying the size of thetotal FOV in accordance with the embodiment in FIG. 6A.

FIGS. 7A-7C are illustrations of three configurations of the movableobjective lens in FIG. 6A in accordance with another three embodimentsof the present invention.

FIGS. 8A and 8B are schematic illustrations of another twoconfigurations of the new multi-beam apparatus in accordance withanother two embodiments of the present invention.

FIG. 9A is a schematic illustration of tilting the plural beamlets inthe conventional multi-beam apparatus in FIG. 1B in accordance withanother embodiment of the present invention.

FIG. 9B is a schematic illustration of tilting the plural beamlets inthe new multi-beam apparatus in FIG. 5A in accordance with anotherembodiment of the present invention.

FIG. 10 is a schematic illustration of another configuration of the newmulti-beam apparatus in accordance with another embodiment of thepresent invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Various example embodiments of the present invention will now bedescribed more fully with reference to the accompanying drawings inwhich some example embodiments of the invention are shown. Withoutlimiting the scope of the protection of the present invention, all thedescription and drawings of the embodiments will exemplarily be referredto an electron beam. However, the embodiments are not used to limit thepresent invention to specific charged particles.

In the drawings, relative dimensions of each component and among everycomponent may be exaggerated for clarity. Within the followingdescription of the drawings, the same or like reference numbers refer tothe same or like components or entities, and only the differences withrespect to the individual embodiments are described.

Accordingly, while example embodiments of the invention are capable ofvarious modifications and alternative forms, embodiments thereof areshown by way of example in the drawings and will herein be described indetail. It should be understood, however, that there is no intent tolimit example embodiments of the invention to the particular formsdisclosed, but on the contrary, example embodiments of the invention areto cover all modifications, equivalents, and alternatives falling withinthe scope of the invention.

In this invention, “axial” means “in the optical axis direction of anelectron optics element (such as a round lens or a multipole lens), oran imaging system or an apparatus”, “radial” means “in a directionperpendicular to the optical axis”, “on-axial” means “on or aligned withthe optical axis, and “off-axis” means “not on or aligned with theoptical axis”.

In this invention, “an imaging system is aligned with an optical axis”means “all the electron optics elements (such round lens and multipolelens) are aligned with the optical axis”.

In this invention, X, Y and Z axe form Cartesian coordinate. The opticalaxis of the primary projection imaging system is on the Z-axis, and theprimary electron beam travels along the Z-axis.

In this invention, “primary electrons” means “electrons emitted from anelectron source and incident onto a being-observed or inspected surfaceof a sample, and “secondary electrons” means “electrons generated fromthe surface by the “primary electrons”.

In this invention, “pitch” means an interval between two adjacentbeamlets or beams on a plane.

In this invention, “effective deflection plane of a deflector” means“the plane where the total deflection function of the deflector can beequivalent to happen”.

Based on some conventional multi-beam apparatuses proposed in the CROSSREFERENCE, this invention proposes several methods to configure a newmulti-beam apparatus with a variable total FOV. In the new apparatus,the total FOV can be variable in size, orientation and illuminationangle. To clearly express the methods, the multi-beam apparatus in FIG.1B is taken as an example. For sake of simplification in explanation, inthe new apparatus, only three beamlets are shown but the number ofbeamlets can be anyone. In addition, one of the three beamlets ison-axis, but they can all be off-axis. In addition, the elements notrelated to the methods, such as the deflection scanning unit and thebeam separator, are not shown or even not mentioned in the illustrationsand the description of the embodiments.

In each of those conventional multi-beam apparatuses, the pluralbeamlets are deflected towards the optical axis by the image-formingmeans. The deflection angles of the plural beamlets are set to minimizethe off-axis aberrations of the plural probe spots due to the objectivelens. Accordingly the plural deflected beamlets typically pass throughor approach the front focal point of the objective lens, i.e. forming anon-axis crossover on or close to the front focal plane of the objectivelens. The pitches of the plural probe spots therefore depend on thedeflection angles of the plural beamlets and the first (or object) focallength of the objective lens. Hence the pitches can be varied bychanging the deflection angles and/or the first focal length of theobjective lens. For example, in FIG. 1A or FIG. 1B, the deflectionangles α₂ and α₃ of the two off-axis beamlets 102_2 and 102_3 are set tominimize the off-axis aberrations of the probe spots 102_2s and 102_3sdue to the objective lens 131. Accordingly the beamlets 102_2 and 102_3typically pass through or approach the front focal point of theobjective lens 131, i.e. forming the crossover CV on the optical axisand on or close to the front focal plane of the objective lens 131. Thepitch Ps between the probe spots 102_1s and 102_2s is determined by thedeflection angle α2 and the first focal length f of the objective lens131, and can be simply expressed as Ps≈α₂·f. Similarly, the pitch Psbetween the probe spots 102_1s and 102_3s can be simply expressed asPs≈α₃·f.

FIG. 3A, FIG. 4A and FIG. 5A show three embodiments 300A, 400A and 500Aof the new apparatus changing the pitches by varying the deflectionangles, and FIG. 6A shows one embodiment 600A changing the pitches byvarying the first focal length. In the embodiment 300A, thesource-conversion unit 320 comprises one beamlet-limit means 121 withthree beam-limit opening 121_1, 121_2 and 121_3, and one movableimage-forming means 322 with three electron optics elements 322_1, 322_2and 322_3. The effective deflection plane 322_0 of the movableimage-forming means 322 can be moved along the optical axis 300_1 withinthe variation range 322_0r. The pitches of the plural probe spots willbecome large as the effective deflection plane 322_0 is moved close tothe objective 131 and vice versa.

FIG. 3B shows the paths of three beamlets 102_1, 102_2 and 102_3 whenthe effective deflection plane 322_0 is on the position D1. The movablecondenser lens 210 collimates the primary electron beam 102 to benormally incident onto the source-conversion unit 320. The beam-limitopenings 121_1˜121_3 divide the primary electron beam 102 into oneon-axis beamlet 102_1, and two off-axis beamlets 102_2 and 102_3. Thetwo off-axis electron optics elements 322_2 and 322_3 deflect thebeamlets 102_2 and 102_3 respectively towards the optical axis 300_1.The three beamlets 102_1˜102_3 are focused onto the sample surface 7 bythe objective lens 131 and therefore form three probe spots 102_1s,102_2s and 102_3s respectively. The deflection angles α₂ and α₃ of thebeamlets 102_2 and 102_3 are set to minimize the off-axis aberrations ofthe probe spots 102_2s and 102_3s. Accordingly the beamlets 102_2 and102_3 pass through or approach the front focal point of the objectivelens 131, i.e. forming a crossover on the optical axis 300_1 and on orclose to the front focal plane thereof. The two pitches formed by theprobe spots 102_1s˜102_3s are approximately equal to α₂·f and α₃·frespectively. The f is the first focal length of the objective lens 131.The effective deflection plane 322_0 in FIG. 3C is on the position D2closer to the objective 131 than the position D1. Accordingly, thedeflection angles α₂ and α₃ are increased and the two pitches becomelarger. The probe spots 102_2s and 102_3s move outside from the previouspositions (dash line) in FIG. 3B.

In the embodiment 400A in FIG. 4A, the source-conversion unit 420comprises one more image-forming means 124 in comparison with FIG. 1B.The image-forming means 124 with three electron optics elements 124_1,124_2 and 124_3 is below the imaging-forming means 122, and can be movedin a radial direction. Accordingly the source-conversion unit 420 worksin two modes. In the first mode as shown in FIG. 4B, the image-formingmeans 122 is used to form three first virtual images of the singleelectron source 101, and the image-forming means 124 is moved outsidethe path of the beamlets 102_1˜102_3. In the second mode as shown inFIG. 4C, the image-forming means 122 is switched off, and theimage-forming means 124 is moved back to form three first virtual imagesof the single electron source 101. The deflection angles α₂ and α₃ ofthe beamlets 102_2 and 102_3 are smaller in the first mode than in thesecond mode, and accordingly the two pitches are larger in the secondmode. In FIG. 4C, the probe spots 102_2s and 102_3s move outside fromthe previous positions (dash line) in FIG. 4B.

The embodiment 500A in FIG. 5A employs one transfer lens 533 and onefield lens 534 between the source-conversion unit 220 and the objectivelens 131 in comparison with FIG. 1B. Accordingly the transfer lens 533,the field lens 534 and the objective lens 131 constitute the primaryprojection imaging system. FIG. 5B shows the paths of the three beamlets102_1˜102_3. The movable condenser lens 210 collimates the primaryelectron beam 102 to be normally incident onto the source-conversionunit 220. The beam-limit openings 121_1˜121_3 divide the primaryelectron beam 102 into one on-axis beamlet 102_1, and two off-axisbeamlets 102_2 and 102_3. The two off-axis electron optics elements122_2 and 122_3 deflect the beamlets 102_2 and 102_3 respectivelytowards the optical axis 500_1. Consequently three first virtual imagesof the single electron source 101 are formed. Then the transfer lens 533focuses the three beamlets 102_1˜102_3 onto the intermediate image planePP1, i.e. projecting the three first virtual images thereon. Accordinglythree second real images 102_1m, 102_2m and 102_3m of the singleelectron source 101 are formed. The field lens 534 is located at theintermediate image plane PP1, and bends the off-axis beamlets 102_2 and102_3 toward the optical axis 500_1 without influencing the focussituations thereof. After that, the objective lens 131 focuses the threebeamlets 102_1˜102_3 onto the sample surface 7, i.e. projecting thethree second real images 102_1m˜102_3m thereon. Consequently, on thesample surface 7, the three beamlets 102_1˜102_3 form three probe spots102_1s, 102_2s and 102_3s respectively.

In FIG. 5B, the bending angles γ₂ and γ₃ of the beamlets 102_2 and 102_3due to the field lens 534 are set to minimize the off-axis aberrationsof the probe spots 102_2s and 102_3s, and the beamlets 102_2 and 102_3accordingly pass through or approach the front focal point of theobjective lens 131, i.e. forming a crossover CV on the optical axis500_1 and on or close to the front focal plane thereof. The pitch Psbetween the probe spots 102_1s and 102_2s is determined by the bendingangle γ₂ and the first focal length f of the objective lens 131, and canbe simply expressed as Ps≈γ₂·f. Similarly, the pitch Ps between theprobe spots 102_1s and 102_3s can be simply expressed as Ps≈γ₃·f. Thebending angles γ₂ and γ₃ change with the radial shifts of the secondreal images 102_2m and 102_3m, and the radial shifts change with thedeflection angles α₂ and α₃ of beamlets 102_2 and 102_3 due to theelectron optics elements 122_2 and 122_3. Therefore the two pitches canbe varied by adjusting the deflection angles α₂ and α₃. In FIG. 5C, thedeflection angles α₂ and α₃ are larger than in FIG. 5B. Consequently thetwo off-axis probe spots 102_2S and 102_3S are moved away from theprevious positions (shown in dash line) in FIG. 5B to the currentpositions, and the pitches become larger.

In the embodiment 600A in FIG. 6A, the first principal plane 631_2 ofthe objective lens 631 can be shifted along the optical axis 600_1within the variation range 631_2r. The axial shift can be done bymechanically moving the position of the objective lens 631 orelectrically changing the shape and/or position of the objective lensfield. As the first principal plane is closer to the sample surface 7,the first focal length f will become small and the first focal planewill move toward the surface 7. In addition, as the first focal planemoves down, the deflection angles of the plural beamlets decrease.Accordingly the pitches of the plural probe spots will decrease.

FIGS. 6B and 6C show the paths of the three beamlets when the firstprincipal plane 631_2 is respectively on the position D3 and theposition D4. The position D3 is closer to the sample surface 7 than theposition D4. Accordingly, the first focal length f and the deflectionangles α₂ and α₃ of the beamlets 102_2 and 102_3 in FIG. 6B are smallerthan in FIG. 6C. In FIG. 6C, the probe spots 102_2s and 102_3s moveoutside from the previous positions (dash line) in FIG. 6B, and the twopitches become larger than in FIG. 6B.

The objective lens in one conventional multi-beam apparatus is anelectromagnetic compound lens, as one embodiment 131-1 shown in FIG. 1C.The objective lens comprises one magnetic lens and one electrostaticlens, and works in a retarding mode (the landing energy of an electronis lower than the energy of the electron passing through the objectivelens) due to low geometric aberrations and low radiation damage on thesample. The magnetic lens is configured by the coil 131_c1 and the yoke131_y1 with the pole-pieces 131_mp1 and 131_mp2, and the electrostaticlens is formed by the pole-piece 131_mp1, the field-control electrode131_e1 and the sample 8. The potential of the inner pole-piece 131_mp1is higher than the sample 8. The potential of the field-controlelectrode 131_e1 is set to control the electric field on the samplesurface. The electric field can ensure the sample free of electricalbreakdown, reduce the geometric aberrations of the plural probe spots,control the charge-up on the sample surface 7 by reflecting back a partof secondary electrons or enhance the collection of secondary electronbeams. In FIG. 1C, the shape of the magnetic field is not variable, andthe shape of the electrostatic field can only be changed within alimited range. Hence the conventional objective lens is almost notelectrically (changing the potentials of the electrodes and/or theexcitation current of the coil) movable.

Next three solutions for configuring the movable objective lens 631 areproposed in terms of the conventional objective lens 131-1 in FIG. 1C,and respectively shown in FIGS. 7A, 7B and 7C. In FIG. 7A, theembodiment 631-1 comprises one more electrode 631-1_e2 between the innerpole-piece 131_mp1 and the field-control electrode 131_e1 in comparisonwith FIG. 1C. Accordingly the electrostatic lens is formed by the innerpole-piece 131_mp1, the electrode 631-1_e2, the field-control electrode131_e1 and the sample 8. The electrostatic field shape of theelectrostatic lens can be varied by adjusting the potential of theelectrode 131_e1 and the potential of the electrode 631-1_e2 as well. Asthe potential of the electrode 631-1_e2 is adjusted to approach thepotential of the inner pole-piece 131_mp1, the electrostatic field issqueezed towards the sample, which is equal to moving the objective lens631-1 towards the sample 8. Accordingly the electrode 631-1_e2 can becalled as a field-moving electrode.

In FIG. 7B, the embodiment 631-2 comprises two more electrodes 631-2_e2and 631-2_e3 inside the bore of the yoke 131_y1 and above thefield-control electrode 131_e1 in comparison with FIG. 1C. Accordinglythe electrostatic lens is formed by the electrodes 631-2_e3 and631-2_e2, the field-control electrode 131_e1 and the sample 8. Thepotential of the electrodes 631-2_e3 is higher than the sample and canbe equal to the inner pole-piece 131_mp1. The electrostatic field shapeof the electrostatic lens can be varied by adjusting the potential ofthe electrode 131_e1 and the potential of the electrode 631-2_e2 aswell. Similar to FIG. 7A, as the potential of the electrode 631-2_e2 isadjusted to approach the potential of the electrodes 631-1_e3, theelectrostatic field is squeezed towards the sample 8, which is equal tomoving the objective lens 631-2 towards the sample 8. Accordingly theelectrode 631-2_e2 can be called as a field-moving electrode. Incomparison the embodiment 631-1 with FIG. 7A, the magnetic lens can beplaced closer to the sample 8, and thereby providing a deeper magneticimmersion to the sample so as to generate lower aberrations.

In FIG. 7C, the embodiment 631-3 comprises one more coil 631-3_c2 andone more yoke 631-3y2 inside the bore of the yoke 131_y1 and above theinner pole-piece 131_mp1 in comparison with FIG. 1C. Accordingly onelower magnetic lens, one upper magnetic lens and one electrostatic lensare formed. The lower magnetic lens generates one lower magnetic fieldby the coil 131_c1 through the lower magnetic-circuit gap G1 between theinner and outer pole-pieces 131_mp1 and 131_mp2 of the yoke 131_y1,while the upper magnetic lens forms one upper magnetic field by the coil631-3_c2 through the magnetic-circuit gap G2 between the innerpole-piece 131_mp1 and the upper pole-piece 631-3_mp3 of the yoke631-3_y2. The electrostatic lens is formed by the inner pole-piece131_mp1, the field-control electrode 131_e1 and the sample 8. Thedistribution shape of the total magnetic field of the objective lens631-3 changes with the combination of the upper and lower magneticfields, therefore can be varied by adjusting the excitation ratio of theupper and lower magnetic lenses or the current ratio of the coils 131_c1and 631-3_c2. As the current ratio is adjusted higher, the totalmagnetic field of the objective lens 631-3 is squeezed towards thesample, which is equal to moving the objective lens 631-3 towards thesample 8. Two extreme examples are the total magnetic field of theobjective lens 631-3 locates the superior top when the coil 131_c1 turnsoff and the coil 631-3-c2 turn on, and the lowest when the coil 131_c1turns on and the coil 631-3_c2 turns off. Each of the solutions in FIGS.7A and 7B can be combined with the solution in FIG. 7C to configure moreembodiments of the movable objective lens 631.

Next some methods of intentionally rotating the probe spot array will beproposed, which can be used to eliminate the orientation variation ofthe total FOV with respect to changes in the observing conditions and/oraccurately match the orientations of sample patterns and the probe spotarray. As mentioned above, the objective lens in one conventionalmulti-beam apparatus is typically an electromagnetic compound lens, suchas the embodiment 131-1 shown in FIG. 1C. Therefore appropriatelycombining the focusing powers of the magnetic lens and the electrostaticlens can rotate the probe spot array around the optical axis to acertain degree. For example, if the objective lens 131 in FIG. 1A issimilar to the embodiment 131-1 in FIG. 1C, the field-control electrode131_e1 can be used to control the rotation of the probe spots 102_2 and102_3s to a certain degree as well as controlling the electrical fieldon the surface 7. To keep the electrical field on the surface 7 weakerthan a permissible value for the specimen safety, the potential of thefield-control electrode 131_e1 can be varied within one specific range,such as −3 kV˜5 kV with respect to the sample 8. The focusing power ofthe electrostatic lens changes with the potential of the field-controlelectrode 131_e1, and accordingly the focusing power of the magneticlens needs being changed to keep the plural beamlets focused on thesample surface 7. The focusing power variation of the magnetic lenschanges the rotation angles of the probe spots 102_2s and 102_3s. Hence,the rotation angles of the probe spots 102_2s and 102_3s can be adjustedby varying the potential of the field-control electrode 131_e1 withinthe specific range.

For each of the foregoing embodiments 300A, 400A and 500A of the newapparatus in FIGS. 3A, 4A and 5A, if the objective lens 131 has aconfiguration similar to the embodiment 131-1, the orientation of theprobe spot array can be adjusted by this method. For the embodiment 600Aof the new apparatus in FIG. 6A, if the movable objective lens 631 has aconfiguration similar to one of the embodiments 631-1, 631-2 and 631-3in FIGS. 7A-7C, the field-control electrode 131_e1 and/or thecorresponding field-moving electrode can be used to control the rotationof the probe spot array. For the embodiment 631-3, the orientation canalso be changed by varying the polarities of the magnetic fields of theupper magnetic lens and the lower magnetic lens. As well known, for amagnetic lens, the rotation angle is related to the polarity of themagnetic field but the focusing power is not. When the polarities of themagnetic fields of the upper magnetic lens and the lower magnetic lensare same, the upper magnetic lens and the lower magnetic lens rotate theprobe spot array in a same direction. When the polarities are oppositeto each other, the upper magnetic lens and the lower magnetic lensrotate the probe spot array in opposite directions. Hence the embodiment631-3 can generate two different orientations of the probe spot arraywith respect to a required focusing power and the corresponding positionof the first principal plane.

For the embodiment 500A in FIG. 5A, the transfer lens 533 and the fieldlens 534 provide more possibilities to control the rotation of the probespot array. One embodiment 510A is shown in FIG. 8A, wherein theelectromagnetic compound transfer lens 533-1 comprises one electrostatictransfer lens 533_11 and one magnetic transfer lens 533_12. The magneticfield of the magnetic transfer lens 533_12 can be adjusted to change therotation of the probe spot array, and the electrostatic field of theelectrostatic transfer lens 533_11 can be accordingly varied to keep thethree second real images 102_1m, 102_2m and 102_3m on the intermediateimage plane PP1. Another embodiment 520A is shown in FIG. 8B, whereinthe electromagnetic compound field lens 534-1 comprises oneelectrostatic field lens 534_11 and one magnetic field lens 534_12. Themagnetic field of the magnetic field lens 534_12 can be adjusted tochange the rotation of the probe spot array, and the electrostatic fieldof the electrostatic field lens 534_11 can be accordingly varied togenerate the required bending angles of the plural beamlets.

In each of the foregoing embodiments, the plural beamlets are normal orsubstantially normal incident onto the sample surface, i.e. the incidentangles or landing angles (angles formed with the normal of the samplesurface) of the plural beamlets are approximately equal to zero. Toeffectively observe some patterns of a sample, the incident angles arebetter a little larger than zero. In this case, to ensure pluralbeamlets perform alike, the plural beamlets are required to have sameincident angles. To do so, the crossover CV of the plural beamles needsto be shifted away from the optical axis. The shift of the crossover CVcan be done by the image-forming means or one additional beamlet-tiltingdeflector.

FIG. 9A shows how to tilt the plural beamlets 102_1˜102_3 by theimage-forming means 122 in the conventional multi-beam apparatus 200A.In comparison with FIG. 1B, the deflection angles α1 (equal to zero inFIG. 1B), α2 and α3 of the beamlets 102_1˜102_3 respectively are addedsame or substantially same amounts so that the crossover CV of thebeamlets 102_1˜102_3 is shifted away from the optical axis 100_1 and onor close to the first focal plane of the objective lens 131. Accordinglythe beamlets 102_1˜102_3 obliquely land on the surface 7 with same ornearly same landing angles. The plural beamlets 102_1˜102_3 in each ofthe embodiments 300A, 400A, 500A and 600A can be tilted by thecorresponding image-forming means in the same way. For the embodiments300A, 400A and 600A, the paths of plural beamlets 102_1˜102_3 will besimilar to those in FIG. 9A. For the embodiment 500A, the paths will bedifferent, as shown in FIG. 9B. In comparison with FIG. 5B, thedeflection angles α1 (equal to zero in FIG. 5B), α2 and α3 of thebeamlets 102_1˜102_3 shift the three second real images 102_1m, 102_2mand 102_3m same or substantially distances on the intermediate imageplane PP1. Accordingly the crossover CV of the beamlets 102_1˜102_3after bended by the field 534, is still on or close to the first focalplane but shifted away from the optical axis 500_1.

FIG. 10 shows how to tilt the plural beamlets 102_1˜102_3 by onebeamlet-tilting deflector 135 in one embodiment 700A of the newapparatus. In comparison with FIG. 1B, the beamlet-tilting deflector 135deflects the beamlets 102_1˜102_3 together to shift the crossover CVaway from the optical axis 700_1 and on or close to the front focalplane of the objective lens 131. Similarly, one beamlet-tiltingdeflector can also be added to the embodiments 300A, 400A, 500A and 600Afor tilting the plural beams together. The beamlet-tilting deflector canbe placed between the source-conversion unit and the front focal planeof the objective lens, and is preferred close to the source-conversionunit. In addition, if the deflection scanning unit in one of theforegoing embodiments is above the front focal plane of the objectivelens, it can shift the crossover of the plural beamlets and theadditional beamlet-tilting deflector therefore is not needed.

Although each of the foregoing embodiments of the new apparatus onlyemploys one or two of the methods for varying the total FOV in size,orientation and incident angle, the methods can be combined in manyways. For example the new apparatus can use one movable image-formingmeans and one movable objective lens together, or use one movableobjective lens, one transfer lens and one field lens together. Althoughthe methods are shown and explained by taking the embodiment 200A inFIG. 1B as an example, the methods can be applied to the otherembodiments (such as the embodiment 100A in FIG. 1A) of the conventionalapparatuses to configure more embodiments of the new multi-beamapparatus.

In summary, based on the conventional multi-beam apparatuses proposed inthe CROSS REFERENCE, this invention proposes several methods toconfigure a new multi-beam apparatus whose total FOV is variable insize, orientation and incident angle. Hence the new apparatus providesmore flexibility to speed the sample observation and enable more kindsof samples observable. More specifically, the new apparatus can be usedas a yield management to provide more possibilities to achieve a highthroughput and detect more kinds of defects. Three methods are proposedto change the pitches of the plural beamlets on the sample surface forvarying the size of the total FOV, i.e. using a movable image-formingmeans in the source-conversion unit, using a movable objective lens, andusing a transfer lens and a field lens between the source-conversionunit and the objective lens. Three methods are employed to intentionallyrotate the probe spot array for varying the orientation of the totalFOV, i.e. using an electromagnetic compound objective lens and varyingthe electric field thereof, using one objective lens with two magneticlenses and setting the magnetic fields thereof opposite in polarity, andusing one magnetic lens in either or both of the transfer lens and thefield lens. Three methods are proposed to shift the crossover of theplural beamlets away from the optical axis for equally varying thelanding angles of the plural beamlets on the sample surface. The shiftcan be done by the image-forming means, or one additionalbeamlet-tilting deflector, or the deflection scanning unit.

Although the present invention has been explained in relation to itspreferred embodiment, it is to be understood that other modificationsand variation can be made without departing the spirit and scope of theinvention as hereafter claimed.

What is claimed is:
 1. A multi-beam apparatus for observing a surface ofa sample, comprising: an electron source; a condenser lens below saidelectron source; a source-conversion unit below said condenser lens; anobjective lens below said source-conversion unit; a deflection scanningunit below said source-conversion unit; a sample stage below saidobjective lens; a beam separator below said source-conversion unit; asecondary projection imaging system; and an electron detection devicewith a plurality of detection elements, wherein said electron source,said condenser lens and said objective lens are aligned with a primaryoptical axis of said apparatus, and said sample stage sustains saidsample so that said surface faces to said objective lens, wherein saidsource-conversion unit comprises a beamlet-limit means with a pluralityof beam-limit openings, and an image-forming means with a plurality ofelectron optics elements and movable along said primary optical axis,wherein said electron source generates a primary-electron beam alongsaid primary optical axis and said condenser lens focuses saidprimary-electron beam, wherein a plurality of beamlets of saidprimary-electron beam pass through said plurality of beam-limit openingsrespectively, and is deflected by said plurality of electron opticselements towards said primary optical axis to form a plurality ofvirtual images of said electron source respectively, wherein saidplurality of beamlets is focused by said objective lens onto saidsurface and therefore forms a plurality of probe spots thereonrespectively, and said deflection scanning unit deflects said pluralityof beamlets to scan said plurality of probe spots respectively over aplurality of scanned regions within an observed area on said surface,wherein a plurality of secondary electron beams is generated by saidplurality of probe spots respectively from said plurality of scannedregions and directed into said secondary projection imaging system bysaid beam separator, said secondary projection imaging system focusesand keeps said plurality of secondary electron beams to be detected bysaid plurality of detection elements respectively, and each detectionelement therefore provides an image signal of one corresponding scannedregion.
 2. The apparatus according to claim 1, wherein deflection anglesof said plurality of beamlets due to said plurality of electron opticselements are respectively set to reduce off-axis aberrations of saidplurality of probe spots.
 3. The apparatus according to claim 2, whereinpitches of said plurality of probe spots are varied together by movingsaid image-forming means along said primary optical axis.
 4. Theapparatus according to claim 2, wherein said objective lens comprises amagnetic lens and an electrostatic lens.
 5. The apparatus according toclaim 4, wherein an orientation of said plurality of probe spots isselectable by varying a ratio of focusing powers of said magnetic lensand said electrostatic lens.
 6. The apparatus according to claim 2,wherein said deflection angles ensure said plurality of beamlets to landon said surface perpendicularly or substantially perpendicularly.
 7. Theapparatus according to claim 2, wherein said deflection angles ensuresaid plurality of beamlets to obliquely land on said surface with sameor substantially same landing angles.
 8. The apparatus according toclaim 6, wherein said deflection scanning unit is above a front focalplane of said objective lens.
 9. The apparatus according to claim 8,wherein said deflection scanning unit tilts said plurality of beamletsto obliquely land on said surface with same or substantially samelanding angles.
 10. The apparatus according to claim 6, furthercomprising a beamlet-tilting deflector between said source-conversionunit and a front focal plane of said objective lens.
 11. The apparatusaccording to claim 10, wherein said beamlet-tilting deflector tilts saidplurality of beamlets to obliquely land on said surface with same orsubstantially same landing angles.
 12. A multi-beam apparatus forobserving a surface of a sample, comprising: an electron source; acondenser lens below said electron source; a source-conversion unitbelow said condenser lens; an objective lens below saidsource-conversion unit; a deflection scanning unit below saidsource-conversion unit; a sample stage below said objective lens; a beamseparator below said source-conversion unit; a secondary projectionimaging system; and an electron detection device with a plurality ofdetection elements, wherein said electron source, said condenser lensand said objective lens are aligned with a primary optical axis of saidapparatus, and said sample stage sustains said sample so that saidsurface faces to said objective lens, wherein said source-conversionunit comprises a beamlet-limit means with a plurality of beam-limitopenings, a first image-forming means with a plurality of first electronoptics elements and a second image-forming means with a plurality ofsecond electron optics elements, said second image-forming means isbelow said first image-forming means and movable in a radial direction,and one of said first image-forming means and said second image-formingmeans is used as an active image-forming means, wherein said electronsource generates a primary-electron beam along said primary optical axisand said condenser lens focuses said primary-electron beam, wherein aplurality of beamlets of said primary-electron beam pass through saidplurality of beam-limit openings respectively, and is deflected by saidactive image-forming means towards said primary optical axis to form aplurality of virtual images of said electron source respectively,wherein said plurality of beamlets is focused by said objective lensonto said surface and therefore forms a plurality of probe spots thereonrespectively, and said deflection scanning unit deflect said pluralityof beamlets to scan said plurality of probe spots respectively over aplurality of scanned regions within an observed area on said surface,wherein a plurality of secondary electron beams is generated by saidplurality of probe spots respectively from said plurality of scannedregions and directed into said secondary projection imaging system bysaid beam separator, said secondary projection imaging system focusesand keeps said plurality of secondary electron beams to be detected bysaid plurality of detection elements respectively, and each detectionelement therefore provides an image signal of one corresponding scannedregion.
 13. The apparatus according to claim 12, wherein deflectionangles of said plurality of beamlets due to said active image-formingmeans are respectively set to reduce off-axis aberrations of saidplurality of probe spots.
 14. The apparatus according to claim 13,wherein pitches of said plurality of probe spots are varied together bychanging said active image-forming means between said firstimage-forming means and said second image-forming means, and when saidfirst image-forming means is selected, said second image-forming meansis moved outside so as not to block said plurality of beamlets.
 15. Theapparatus according to claim 13, wherein said objective lens comprises amagnetic lens and an electrostatic lens.
 16. The apparatus according toclaim 15, wherein an orientation of said plurality of probe spots isselectable by varying a ratio of focusing powers of said magnetic lensand said electrostatic lens.
 17. The apparatus according to claim 13,wherein said deflection angles ensure said plurality of beamlets to landon said surface perpendicularly or substantially perpendicularly. 18.The apparatus according to claim 13, wherein said deflection anglesensure said plurality of beamlets to obliquely land on said surface withsame or substantially same landing angles.
 19. The apparatus accordingto claim 17, wherein said deflection scanning unit is above a frontfocal plane of said objective lens.
 20. The apparatus according to claim19, wherein said deflection scanning unit tilts said plurality ofbeamlets to obliquely land on said surface with same or substantiallysame landing angles.
 21. The apparatus according to claim 17, furthercomprising a beamlet-tilting deflector between said source-conversionunit and a front focal plane of said objective lens.
 22. The apparatusaccording to claim 21, wherein said beamlet-tilting deflector tilts saidplurality of beamlets to obliquely land on said surface with same orsubstantially same landing angles.
 23. An multi-beam apparatus forobserving a surface of a sample, comprising: an electron source; acondenser lens below said electron source; a source-conversion unitbelow said condenser lens; an objective lens below saidsource-conversion unit; a deflection scanning unit below saidsource-conversion unit; a sample stage below said objective lens; a beamseparator below said source-conversion unit; a secondary projectionimaging system; and an electron detection device with a plurality ofdetection elements, wherein said electron source, said condenser lensand said objective lens are aligned with a primary optical axis of saidapparatus, a first principal plane of said objective lens is movablealong said primary optical axis, and said sample stage sustains saidsample so that said surface faces to said objective lens, wherein saidsource-conversion unit comprises a beamlet-limit means with a pluralityof beam-limit openings, and an image-forming means with a plurality ofelectron optics elements, wherein said electron source generates aprimary-electron beam along said primary optical axis and said condenserlens focuses said primary-electron beam, wherein a plurality of beamletsof said primary-electron beam pass through said plurality of beam-limitopenings respectively, and is deflected by said plurality of electronoptics elements towards said primary optical axis to form a plurality ofvirtual images of said electron source respectively, wherein saidplurality of beamlets is focused by said objective lens onto saidsurface and therefore forms a plurality of probe spots thereonrespectively, and said deflection scanning unit deflects said pluralityof beamlets to scan said plurality of probe spots respectively over aplurality of scanned regions within an observed area on said surface,wherein a plurality of secondary electron beams is generated by saidplurality of probe spots respectively from said plurality of scannedregions and directed into said secondary projection imaging system bysaid beam separator, said secondary projection imaging system focusesand keeps said plurality of secondary electron beams to be detected bysaid plurality of detection elements respectively, and each detectionelement therefore provides an image signal of one corresponding scannedregion.
 24. The apparatus according to claim 23, wherein deflectionangles of said plurality of beamlets due to said plurality of electronoptics elements are respectively set to reduce off-axis aberrations ofsaid plurality of probe spots.
 25. The apparatus according to claim 24,wherein pitches of said plurality of probe spots are varied together bymoving said first principal plane along said primary optical axis. 26.The apparatus according to claim 24, wherein said deflection anglesensure said plurality of beamlets to land on said surfaceperpendicularly or substantially perpendicularly.
 27. The apparatusaccording to claim 24, wherein said deflection angles ensure saidplurality of beamlets to obliquely land on said surface with same orsubstantially same landing angles.
 28. The apparatus according to claim26, wherein said deflection scanning unit is above a front focal planeof said objective lens.
 29. The apparatus according to claim 28, whereinsaid deflection scanning unit tilts said plurality of beamlets toobliquely land on said surface with same or substantially same landingangles.
 30. The apparatus according to claim 26, further comprising abeamlet-tilting deflector between said source-conversion unit and afront focal plane of said objective lens.
 31. The apparatus according toclaim 30, wherein said beamlet-tilting deflector tilts said plurality ofbeamlets to obliquely land on said surface with same or substantiallysame landing angles.
 32. The apparatus according to claim 24, whereinsaid objective lens comprises a lower magnetic lens and an electrostaticlens.
 33. The apparatus according to claim 32, wherein saidelectrostatic lens comprises a field-control electrode and afield-moving electrode, and generates an electrostatic field.
 34. Theapparatus according to claim 33, wherein a potential of saidfield-control electrode is set to control said electrostatic field onsaid surface for said sample free of electrical breakdown.
 35. Theapparatus according to claim 34, wherein a potential of saidfield-moving electrode is set to move said electrostatic field formoving said first principal plane.
 36. The apparatus according to claim34, wherein an orientation of said plurality of probe spots isselectable by varying either or both of potentials of said field-controlelectrode and said field-moving electrode.
 37. The apparatus accordingto claim 32, further comprising an upper magnetic lens above said lowermagnetic lens.
 38. The apparatus according to claim 37, wherein saidfirst principal plane is moved by varying a ratio of focusing powers ofsaid lower magnetic lens and said upper magnetic lens.
 39. The apparatusaccording to claim 37, wherein an orientation of said plurality of probespots is selectable by setting polarities of magnetic fields of saidupper and lower magnetic lenses same or opposite.
 40. A multi-beamapparatus for observing a surface of a sample, comprising: an electronsource; a condenser lens below said electron source; a source-conversionunit below said condenser lens; a transfer lens below saidsource-conversion unit; a field lens below said transfer lens; anobjective lens below said field lens; a deflection scanning unit belowsaid source-conversion unit; a sample stage below said objective lens; abeam separator below said source-conversion unit; a secondary projectionimaging system; and an electron detection device with a plurality ofdetection elements, wherein said electron source, said condenser lens,said transfer lens, said field lens and said objective lens are alignedwith a primary optical axis of said apparatus, and said sample stagesustains said sample so that said surface faces to said objective lens,wherein said source-conversion unit comprises a beamlet-limit means witha plurality of beam-limit openings, and an image-forming means with aplurality of electron optics elements, wherein said electron sourcegenerates a primary-electron beam along said primary optical axis andsaid condenser lens focuses said primary-electron beam, wherein aplurality of beamlets of said primary-electron beam pass through saidplurality of beam-limit openings respectively, and is deflected by saidplurality of electron optics elements towards said primary optical axisto form a plurality of first virtual images of said electron sourcerespectively, wherein said transfer lens images said plurality of firstvirtual images onto an intermediate image plane and therefore forms aplurality of second real images respectively thereon, said field lens isplaced on said intermediate image plane and bends said plurality ofbeamlets, said objective lens images said plurality of second realimages onto said surface and therefore forms a plurality of probe spotsthereon respectively, and said deflection scanning unit deflects saidplurality of beamlets to scan said plurality of probe spots respectivelyover a plurality of scanned regions within an observed area on saidsurface, wherein a plurality of secondary electron beams is generated bysaid plurality of probe spots respectively from said plurality ofscanned regions and directed into said secondary projection imagingsystem by said beam separator, said secondary projection imaging systemfocuses and keeps said plurality of secondary electron beams to bedetected by said plurality of detection elements respectively, and eachdetection element therefore provides an image signal of onecorresponding scanned region.
 41. The apparatus according to claim 40,wherein bending angles of said plurality of beamlets due to said fieldlens are set to reduce off-axis aberrations of said plurality of probespots.
 42. The apparatus according to claim 41, wherein deflectionangles of said plurality of beamlets due to said plurality of electronoptics elements are adjusted to change pitches of said plurality ofprobe spots respectively.
 43. The apparatus according to claim 41,wherein said objective lens comprises a first magnetic lens and a firstelectrostatic lens.
 44. The apparatus according to claim 43, wherein anorientation of said plurality of probe spots is selectable by varying aratio of focusing powers of said first magnetic lens and said firstelectrostatic lens.
 45. The apparatus according to claim 41, whereinsaid transfer lens comprises a second magnetic lens and a secondelectrostatic lens.
 46. The apparatus according to claim 45, wherein anorientation of said plurality of probe spots is selectable by varying aratio of focusing powers of said second magnetic lens and said secondelectrostatic lens.
 47. The apparatus according to claim 41, whereinsaid field lens comprises a third magnetic lens and a thirdelectrostatic lens.
 48. The apparatus according to claim 47, wherein anorientation of said plurality of probe spots is selectable by varying aratio of focusing powers of said third magnetic lens and said thirdelectrostatic lens.
 49. The apparatus according to claim 41, whereinsaid bending angles and deflection angles of said plurality of beamletsdue to said plurality of electron optics elements ensure said pluralityof beamlets to land on said surface perpendicularly or substantiallyperpendicularly.
 50. The apparatus according to claim 41, wherein saidbending angles and deflection angles of said plurality of beamlets dueto said plurality of electron optics elements ensure said plurality ofbeamlets to obliquely land on said surface with same or substantiallysame landing angles.
 51. The apparatus according to claim 49, whereinsaid deflection scanning unit is above a front focal plane of saidobjective lens.
 52. The apparatus according to claim 51, wherein saiddeflection scanning unit tilts said plurality of beamlets to obliquelyland on said surface with same or substantially same landing angles. 53.The apparatus according to claim 49, further comprising abeamlet-tilting deflector between said source-conversion unit and afront focal plane of said objective lens.
 54. The apparatus according toclaim 53, wherein said beamlet-tilting deflector tilts said plurality ofbeamlets to obliquely land on said surface with same or substantiallysame landing angles.
 55. A method to configure a multi-beam apparatusfor observing a surface of a sample, comprising steps of: configuring animage-forming means of a source-conversion unit movable along a primaryoptical axis thereof, the image-forming means having a plurality ofelectron optics elements; using said image-forming means to form aplurality of virtual images of an electron source respectively; using anobjective lens to image said plurality of virtual images onto saidsurface and form a plurality of probe spots thereon; and moving saidimage-forming means to vary pitches of said plurality of probe spots.56. A method to configure a multi-beam apparatus for observing a surfaceof a sample, comprising steps of: configuring a source-conversion unitwith a first image-forming means and a second image-forming means,wherein said second image-forming means is farther away from an electronsource than said first image-forming means and movable in a radialdirection of said apparatus; using one of said first image-forming meansand said second image-forming means as an active image-forming means,wherein when said first image-forming means is used, said secondimage-forming means is moved away; using said active image-forming meansto form a plurality of virtual images of said electron sourcerespectively; using an objective lens to image said plurality of virtualimages onto said surface and form a plurality of probe spots thereon;and changing said active image-forming means between said firstimage-forming means and said second image-forming means to vary pitchesof said plurality of probe spots.
 57. A method to configure a multi-beamapparatus for observing a surface of a sample, comprising steps of:configuring an objective lens with a first principal plane movable alonga primary optical axis of said apparatus; using an image-forming meansof a source-conversion unit to form a plurality of virtual images of anelectron source respectively; using said objective lens to image saidplurality of virtual images onto said surface and form a plurality ofprobe spots thereon; and moving said first principal plane to varypitches of said plurality of probe spots.
 58. A method to configure amulti-beam apparatus for observing a surface of a sample, comprisingsteps of: configuring an objective lens with a lower magnetic lens andan electrostatic lens in said apparatus; using an image-forming means ofa source-conversion unit to form a plurality of virtual images of anelectron source respectively; using said objective lens to image saidplurality of virtual images onto said surface and form a plurality ofprobe spots thereon; and changing a ratio of focusing powers of saidmagnetic lens and said electrostatic lens to select an orientation ofsaid plurality of probe spots.
 59. The method according to claim 58,further comprising a step of configuring said objective lens with anupper magnetic lens farther away from said surface than said lowermagnetic lens.
 60. The method according to claim 59, further comprisinga step of changing polarities of magnetic fields of said upper and lowermagnetic lenses to select said orientation.
 61. A method to configure amulti-beam apparatus for observing a surface of a sample, comprisingsteps of: using an image-forming means of a source-conversion unit todeflect a plurality of beamlets from an electron source to form aplurality of first virtual images thereof respectively; using anobjective lens to image said plurality of virtual images onto saidsurface and form a plurality of probe spots thereon; and settingdeflection angles of said plurality of beamlets due to saidimage-forming means so that said plurality of beamlets lands on saidsurface with same or substantially same landing angles.
 62. The methodaccording to claim 61, further comprising a step of changing saiddeflection angles to equally vary said landing angles.
 63. The methodaccording to claim 61, further comprising a step of using a deflectionscanning unit to tilt said plurality of beamlets so as to equally varysaid landing angles.
 64. The method according to claim 61, furthercomprising a step of using a beamlet-tilting deflector to tilt saidplurality of beamlets so as to equally vary said landing angles.
 65. Amethod to configure a multi-beam apparatus for observing a surface of asample, comprising steps of: using an image-forming means of asource-conversion unit to deflect a plurality of beamlets from anelectron source to form a plurality of first virtual images thereofrespectively; using a transfer lens to image said plurality of firstvirtual images onto an intermediate image plane and forms a plurality ofsecond real images respectively; placing a field lens on saidintermediate image plane to bend said plurality of beamlets; and usingan objective lens to image said plurality of second real images ontosaid surface and form a plurality of probe spots thereon.
 66. The methodaccording to claim 65, further comprising a step of changing deflectionangles of said plurality of beamlets due to said image-forming means tovary pitches of said plurality of probe spots.
 67. The method accordingto claim 65, further comprising a step of setting deflection angles ofsaid plurality of beamlets due to said image-forming means and bendingangles of said plurality of beamlets due to said field lens so that saidplurality of beamlets lands on said surface with same or substantiallysame landing angles.
 68. The method according to claim 67, furthercomprising a step of varying said deflection angles to equally changesaid landing angles.
 69. The method according to claim 67, furthercomprising a step of using a deflection scanning unit to tilt saidplurality of beamlets to equally change said landing angles.
 70. Themethod according to claim 67, further comprising a step of using abeamlet-tilting deflector to tilt said plurality of beamlets to equallychange said landing angles.
 71. The method according to claim 65,further comprising a step of configuring said objective lens with afirst magnetic lens and a first electrostatic lens.
 72. The methodaccording to claim 71, further comprising a step of changing a ratio offocusing powers of said first magnetic lens and said first electrostaticlens to select an orientation of said plurality of probe spots.
 73. Themethod according to claim 65, further comprising a step of configuringsaid transfer lens with a second magnetic lens and a secondelectrostatic lens.
 74. The method according to claim 73, furthercomprising a step of changing a ratio of focusing powers of said secondmagnetic lens and said second electrostatic lens to select anorientation of said plurality of probe spots.
 75. The method accordingto claim 65, further comprising a step of configuring said field lenswith a third magnetic lens and a third electrostatic lens.
 76. Themethod according to claim 75, further comprising a step of changing aratio of focusing powers of said third magnetic lens and said thirdelectrostatic lens to select an orientation of said plurality of probespots.
 77. An apparatus, comprising: a source for providing a primarycharged particle beam; a source-conversion unit for dividing the primarycharged particle beam into a plurality of charged particle beamlets andusing which to form a plurality of images of the source respectively,the source-conversion unit comprising a beamlet-limit means withbeam-limit openings and an image-forming means with electron opticselements; an objective lens below said source-conversion unit forprojecting the plurality of images onto a sample surface; whereinpitches of the plurality of charged particle beamlets on the samplesurface are adjustable by changing deflection angles of the plurality ofcharged particle beamlets prior entering a distance from a deflectionplane of the source-conversion unit to the objective lens.
 78. Anapparatus, comprising: a source for providing a primary charged particlebeam; a beamlet-limit means with beam-limit openings for usingallowing aplurality of beamlets of the primary charged particle beam to passthrough, the plurality of beamlets being used to form a plurality ofimages of the source; an objective lens for projecting the plurality ofimages onto a sample surface to form a plurality of probe spots; and animage-forming means with electron optics elements for adjusting pitchesof the plurality of probe spots on the sample surface by moving along aprimary optical axis.
 79. A method for observing a sample surface, saidmethod comprising steps of: providing a plurality of charged particlebeams with a plurality of crossovers respectively using asource-conversion unit; projecting the plurality of crossovers onto thesample surface to form a plurality of probe spots thereon using anobjective lens; scanning the plurality of probe spots on the samplesurface; and changing a distance from a deflection angles of theplurality of charged particle beams plane of the source-conversion unitto the objective lens such that pitches of the plurality of probe spotscan be adjusted.
 80. A source-conversion unit comprising: abeamlet-limit device with a plurality of beam-limit openings configuredto allow a plurality of beamlets to pass through: and an image-formingdevice movable along a primary optical axis and comprising a pluralityof electron optics elements, wherein at least some of the plurality ofelectron optics are configured to deflect at least some of the pluralityof beamlets towards the primary optical axis.
 81. The source-conversionunit of claim 80, wherein the deflection of the at least some of theplurality of beamlets towards the primary optical axis form a pluralityof virtual images of an electron source.
 82. The source-conversion unitof claim 80, wherein the at least some of the plurality of electronoptics elements are configured to deflect at least some of the pluralityof beamlets at deflection angles to reduce off-axis aberrations of aplurality of probe spots.
 83. The source-conversion unit of claim 82,wherein the plurality of probe spots have pitches that are variedtogether by moving the image-forming device along the primary opticalaxis.
 84. The source-conversion unit of claim 82, wherein the pluralityof probe spots have an orientation that is selectable by varying a ratioof focusing powers of a magnetic lens and an electrostatic lens.
 85. Thesource-conversion unit of claim 82, wherein the deflection angles enablethe at least some of the plurality of beamlets to land on a surface of asample perpendicularly or substantially perpendicularly.
 86. Thesource-conversion unit of claim 82, wherein the deflection angles enablethe at least some of the plurality of beamlets to obliquely land on asurface of a sample with a same or substantially same landing angles.87. A multi-beam apparatus comprising: an electron source configured togenerate a charged-particle beam along a primary optical axis: asource-conversion unit comprising: a beamlet-limit device with aplurality of beam-limit openings configured to allow a plurality ofbeamlets of the charged-particle beam to pass through, and animage-forming device movable along the primary optical axis andcomprising a plurality of electron optics elements, wherein at leastsome of the plurality of electron optics are configured to deflect atleast some of the plurality of beamlets towards the primary opticalaxis: and an objective lens configured to focus the plurality ofbeamlets on a surface of a sample to form a plurality of probe spots.88. The multi-beam apparatus of claim 87, wherein the deflection of theat least some of the plurality of beamlets towards the primary opticalaxis form a plurality of virtual images of the electron source.
 89. Themulti-beam apparatus of claim 87, wherein the objective lens comprises amagnetic lens and an electrostatic lens.
 90. The multi-beam apparatus ofclaim 89, wherein the magnetic lens and the electrostatic lens areconfigured to enable a selection of an orientation of the plurality ofprobe by varying a ratio of focusing powers of the magnetic lens and theelectrostatic lens.
 91. The multi-beam apparatus of claim 87, whereinthe plurality of probe spots have pitches that are varied together bymoving the image-forming device along the primary optical axis.
 92. Themulti-beam apparatus of claim 87, wherein the at least some of theplurality of electron optics elements are configured to deflect at leastsome of the plurality of beamlets at deflection angles to reduceoff-axis aberrations of the plurality of probe spots.
 93. The multi-beamapparatus of claim 92, wherein the deflection angles enable the at leastsome of the plurality of beamlets to land on a surface of a sampleperpendicularly or substantially perpendicularly.
 94. The multi-beamapparatus of claim 92, wherein the deflection angles enable the at leastsome of the plurality of beamlets to obliquely land on a surface of asample with same or substantially same landing angles.
 95. Themulti-beam apparatus of claim 87, further comprising a deflectionscanning unit positioned above a front focal plan of the objective lens.96. The multi-beam apparatus of claim 95, wherein the deflectionscanning unit is configured to tilt the plurality of beamlets toobliquely land on the surface with a same or substantially the samelanding angles.
 97. The multi-beam apparatus of claim 87, furthercomprising a beamlet-tilting deflector positioned between thesource-conversion unit and a front focal plan of the objective lens. 98.The multi-beam apparatus of claim 97, wherein the beamlet-tiltingdeflector is configured to tilt the plurality of beamlets to obliquelyland on the surface with a same or substantially the same landingangles.
 99. A method to configure a multi-beam apparatus for observing asurface of a sample, the method comprising: forming a plurality ofvirtual images of an electron source using an image-forming device withelectron optics elements, wherein the virtual images are used to form aplurality of probe spots on the surface; and moving the image formingdevice along a primary axis to vary pitches of the plurality of probespots.
 100. The method of claim 99, further comprising forming theplurality of probe spots on the sample using an objective lens to imagethe plurality of virtual images on the surface.